As is well known, punch presses are highly useful for producing various types of cutouts in sheet-like workpieces of metal, plastic, composite materials and the like. In recent years, numerically controlled punch presses have enabled the production of relatively complex cutouts by repetitive punch strokes in nibbling operations, because movement of a workpiece clamping mechanism may be controlled by an X-Y coordinate guidance system.
Moreover, with the advent of large turret assemblies in turret-type punch presses and with the later advent of automatic tool changer type punch presses, a fairly large amount of tooling may be provided within the press so that the operative punch and die may be readily changed. Thus, even complex configurations have been made possible through the optimization of computer numerically controlled systems in presses providing a multiplicity of interchangeable tools.
However, the configuration of cutouts that could be obtained by a punch press still has been dictated by the specific tooling available for operation without changing the tools in the turrets or in the tool changer carriers, and it has generally been necessary to change the tooling in the operative ram position in order to effect any modification in the cutout being produced. More recently in Hirata et al. U.S. Pat. No. 4,412,469 granted Nov. 1, 1983, it has been proposed to provide additional variation in the nature of the cutouts produced by a single set of tooling by providing a rotatable set of tooling in a turret-type punch press. More particularly, at two diametrically spaced positions in the turrets, there has been provided tooling which can be rotated by a pair of servomotors. As a result, rotation of the tooling may be coupled with the indexing of the workpiece to spaced positions to produce cutouts with the tooling rotated relative to the X and Y axes of movement of the workpiece so that the cutouts are at different angular relationships.
Even more recently, there has been introduced a tool changer type punch press in which the punch is rigidly coupled to the lower portion of the ram assembly so as to move integrally therewith and to provide a mechanism which effects rotation of the lower portion of the ram assembly and thereby the punch. Moreover, the die is firmly held in a die holder assembly which is rotated simultaneously with the lower portion of the ram assembly by the same drive mechanism. This punch press is illustrated and described in the copending application of Hans Klingel entitled "Punch Press With Rotary Ram And Method Of Operating Same", Ser. No. 661,399 filed Oct. 16, 1984.
The availability of such rotary action for the punch press tooling provided by these two rotary action presses has thus increased the versatility of a single set of tooling.
In nibbling or contouring operations, the punch tooling repetitively acts upon the workpiece as it is moved relative to the punching station to produce a series of overlapping cutouts which ultimately define a large contoured cutout, and this may have rectilinear and/or curvilinear edges. Generally, the tooling for such nibbling operations has utilized a relatively small diameter, generally cylindrical cutting portion on the punch and a cooperatively configured die, although polygonal cross section punches are also used. As is well known, the size of the punch that may be utilized is limited by the punching force generatable by the punch press, the thickness of the sheet material comprising the workpiece and the shear strength of the material comprising the workpiece. Moreover, the feed rate or rate at which the sheet material is indexed to form the contoured edge on the workpiece will depend upon the amount of overlap of adjacent punch strokes which, in turn, is often dictated by the amount of roughness tolerable in the contoured edge.
As will be appreciated, nibbling of a straight line with a circular punch will produce an edge defined by a series of arcs and the roughness will be dependent upon the feed rate per stroke as seen in FIG. 18a, where E represents the lineal advance of the workpiece or feed rate, D.sub.n =diameter of the nibbling tool, and R.sub.t =roughness or the width of the sector defined by the intersecting arcs of the adjacent punch cutouts.
The feed rate E for a straight edge is usually controlled to limit R.sub.t to 0.2-0.4 mm, determined in accordance with the following formula: ##EQU1## If the allowable roughness is greater and the diameter of the punch can be greater, then the feed rate E can be increased to produce the nibbled cutout faster.
Similarly, if a curved edge is to be nibbled, the feed rate is also dependent upon the amount of roughness as seen in FIG. 18b, where E=represents the lineal advance of the workpiece between strokes, R.sub.z =the radius of the punch, R.sub.s =the radius of the curve for the cutout, and R.sub.t =roughness or the length of the sector defined by the intersecting arcs of the adjacent cutouts. The comparable formula is as follows: ##EQU2##
By use of a punch with an arcuate cutting face having the same radius as that of the desired curve for the cutout, the roughness produced by the nibbling can be eliminated. However, in almost all instances this would require a circular punch with a very large diameter and in turn requiring more power than the press has available to punch through the workpiece.
The force to cut through the workpiece with a circular punch is, for a single stroke cutout, determined in accordance with the formula: EQU F=D.sub.s .multidot..pi..multidot.S.multidot.T.sub.s
where D.sub.s is the diameter of the punch, S is the thickness of the sheet material, and T.sub.s is the shear strength of the sheet material. For the force required using a rectangular punch, the formula is: EQU F=(2 L+2 W).multidot.S.multidot.T.sub.s
where L and W are the length and width of the punch cross section. It can be seen that the force required can rapidly reach and exceed the capacity of the press as thicker materials are employed.
A further factor that must be considered in punch design is that the width of the punch must be at least as great as, and preferably greater than, the thickness of the sheet material. If not, the punch is subject to breakage. Moreover, polygonal punches wear more rapidly in cutting sheet material than does a tool of circular cross section, and this is particularly significant as thicker sheet material is being punched.
It is an object of the present invention to provide a novel punching method for nibbling contoured cutouts in workpieces at a relatively rapid rate with one set, or a limited number of sets, of tooling.
It is also an object to provide such a method in which the tooling will generate contoured edges with relatively low roughness.
Another object is to provide a novel punch press assembly for nibbling contoured cutouts in workpieces at a rapid rate and which includes means for effecting rotation of the tooling and indexing of the workpiece along X and Y axes.
A further object is to provide novel punch tooling for rapidly nibbling contoured cutouts in a workpiece with one set or a limited number of sets, of tooling when used in a punch press which will effect rotation of such tooling and precise indexing of the workpiece.